fix bugs introduced by I3ef1bcdb2eb1c45f68e829ccb6e3ecde28076591

two bugs were introduced by the above CL
1. to test if the column value is NULL, getType_native() should check if
the columnindiex is valid. (it seems sometimes callers could ask if a
given non-existent column is null - and the answer should be true.)
2. if a column is null and isBlob_native, isString_native methods should
return true - not false.

Change-Id: I64df75d0a3840a4502c00db8759c66ba4b268a26